1. Field of the Invention
The present invention relates to a color cathode ray tube with an electron beam selection mask disposed opposite to a color luminescent screen and serving to land each electron beam on a fluorescent pattern of a predetermined color formed on the color luminescent screen.
2. Description of the Prior Art
In the conventional color cathode-ray tube known heretofore, as shown in FIG. 15, an electron beam selection means 2 such as an aperture grille having an array of a multiplicity of vertically elongate slit-like electron beam transmission apertures is disposed in a panel 1 where a color luminescent screen is formed on its inner surface. Such electron beam selection means 2 includes, as shown in FIG. 16 for example, an electron beam selection electrode 3 which is composed of a thin metal plate with an array of vertically elongate slit-like apertures 3a and is attached in a tensed state to a frame 4. The frame 4 comprises a pair of mutually opposed horizontal members 4A, 4B and a pair of vertical arms 4C, 4D interposed between such horizontal members 4A, 4B. And the electron beam selection electrode 3 is disposed with an adequate tension between the frame members 4A and 4B while being anchored at the respective two ends of the individual slit-like apertures 3a.
In general, since the fluorescer pattern of each color is optically printed by the use of such electron beam selection means 2 as an optical mask, it is necessary that the beam selection means 2 be so attached to the panel 1 as to be removable therefrom and reattachable at a predetermined position. Furthermore, when any external impact is applied after completion of the cathode-ray tube as a final product, the electron beam selection mask 2 needs to be returned to the former predetermined position. Normally, therefore, attachment of the beam selection mask 2 is executed by engaging support stud pins 5, which are anchored on peripheral side walls 1s of the panel 1, with leaf springs 6 secured to the frame 4 of the mask 2. For example, the springs 6 are welded at one end thereof to the mutually opposed vertical arms 4C, 4D and the horizontal frame member 4B of the frame 4 respectively, and the stud pins 5 fixed by frit on the inner surface of the panel 1 are fitted into through-holes 7 formed at the respective free ends of the springs 6, whereby the mask 2 is held at a predetermined position detachably.
The stud pin 5 is composed of a ceramic material or the like and is shaped into a truncated cone, and the through-hole 7 of the spring 6 is selectively shaped to have three sides 7A, 7B, 7C which contact the peripheral surface of the stud pin 5 as shown in FIGS. 17 and 18, so that the stud pin 5 is kept in contact with the inner periphery of the through-hole 7 at three points PA, PB, PC on the sides 7A, 7B, 7C respectively, whereby the positions of engagement between the stud pin 5 and the spring 6 are established.
Due to the constitution mentioned above, in case the cathode-ray tube becomes dimensionally greater as observed in the recent trend of production, the electron beam selection mask 2 is also rendered larger in both size and weight, hence raising a problem with regard to reliability. That is, when any great external impact is applied to such a structure, there is induced a positional deviation between the stud pin 5 and the support spring 6 and, even after removal of the impact, the positional relationship between the above two components fails to resume the former stable state prior to the impact and the positional deviation still remains as a result. This phenomenon will be described below in further detail. Suppose now that in FIG. 17, the support spring 6 is inclined, by an external impact, with respect to the stud pin 5 from a line passing through one contact point PC and the center axis 0 of the stud pin 5. If the stud pin 5 is conical in this case, a local shock is caused at the other contact point PA or PB where a displacement is induced in the direction to thrust toward a larger-diameter base portion of the stud pin 5. Meanwhile, if the stud pin 5 is columnar with its portions having an equal diameter, a local shock is caused at the two points PA and PB. Particularly when the electron beam selection mask 2 is great in both size and weight as mentioned above, the stud pin 5 is also shaped to have a larger diameter, so that the distance from the center axis 0 of the stud pin 5 to the point PA or PB becomes longer to consequently increase the moment even if the coefficient of friction remains constant. And since the impact is rendered greater with increase of the weight, it follows that the aforesaid local shock also becomes greater. Thus, even after release from the external impact, there occurs a trouble that the inclination of the support spring 6 is not eliminated because of the friction, or even after successful elimination of the inclination, the inner periphery of the through-hole 7 in the support spring 6 is worn or damaged by the aforementioned local shock and is thereby deformed in the contour, so that the positional relationship between the stud pin 5 and the through-hole 7 in the support spring 6 fails to resume the former state or position prior to application of the impact.
Once such positional deviation is thus induced between the stud pin 5 and the through-hole 7 of the support spring 6, it causes a positional error in the relationship between the electron beam selection mask 2 and the color luminescent screen to eventually bring about disadvantages including occurrence of mislanding of the electron beam which generates color discrepancy.
Furthermore, in the constitution where the beam selection electrode in the color cathode-ray tube has an array of slit-like electron beam transmission apertures 3a extending in one direction (hereinafter referred to as Y-direction) as described above, there may occur a trouble that some external vibration based on the sound from a speaker or the like is imparted to the beam selection electrode 3 to vibrate the same, thereby causing "shake" in the electron beam transmission apertures 3a to eventually deteriorate the picture quality.
It is customary in the prior art that the harmful influence of such vibration in the electron beam selection electrode 3 is diminished by increasing both the mechanical strength of the frame 4 and the tension for support of the electrode 3 to render the vibration damping characteristic steep.
Besides the above, in the color cathode-ray tube equipped with such electron beam selection mask 2, the beam path is limited by the beam selection electrode 3 during the operation so that the electron beam fails to pass through the aperture 3a and consequently the electrons come to impinge upon the electrode 3 itself. As a result, the beam selection mask 2 is heated up to a considerably high temperature to cause thermal expansion of the beam selection electrode 3, whereby an error is induced relative to the landing position of the electron beam on the luminescent screen. This phenomenon will be described below with reference to FIG. 19. Suppose now that a single electron beam 8 corresponding to one color such as red passes through a slit-like beam transmission aperture 3a in the beam selection electrode 3 shown by solid lines and lands on a striped red fluorescer pattern 9R of the color luminescent screen 9 formed on a front surface 1f of the panel 1. If the structure is so designed that, at normal temperature, the electron beam 8 lands exactly on the fluorescer pattern 9R through the slit-like beam transmission aperture 3a in the beam selection electrode 3, then, as mentioned previously, the electron beam selection mask 2 is heated during the operation of the cathode-ray tube and is thereby expanded thermally, so that the electron beam transmission aperture 3a is displaced horizontally in the direction of arrows 11a and 11b as shown by chain lines in FIG. 19. Consequently, the landing position of the electron beam on the luminescent screen 9 deviates from the relevant pattern 9R and varies to a fluorescer pattern of another color as indicated by chain-line arrows to eventually cause color discrepancy. This phenomenon is conspicuous particularly in the peripheral regions of the screen where the displacement resulting from the thermal expansion is great. For averting such disadvantage, the space termed "bar height" between the electrode 3 and the luminescent screen 9 is changed with the thermal expansion of the electrode 3 from an initial bar height h1 to a required small bar height h2 as shown by broken lines in FIG. 19 in accordance with the thermal expansion or temperature rise, whereby the displacement to the landing position on the luminescent screen 9 can be avoided.
Relative to the constitution described in connection with FIG. 15, there is known on method of changing the bar height or the space between the electrode and the luminescent screen in accordance with the temperature rise, wherein each of the springs employed to support the frame 4 of the electron beam selection electrode 3 is formed into a bimetal structure so that the temperature can be sensed therefrom for adjustment of the space between the electrode and the luminescent screen. However, since the above known technique is not capable of discriminating between the ambient temperature variation and the electrode temperature variation, there is a problem that even when substantially no change is existent in the positional relationship between the electrode 3 and the luminescent screen 9, the space therebetween is changed indiscriminatively to induce a color discrepancy if the sensed temperature variation is the ambient one.